feat: switch wizard to CIMD URL as client_id for automatic multi-region support

[MattBro]

Problem

The wizard currently uses a hardcoded UUID client_id (019a0c79-b69d-0000-f31b-b41345208c9d) which only exists on US. EU requires manually creating a matching OAuthApplication in admin.

Solution

Switch the wizard's client_id to its CIMD metadata URL: https://us.posthog.com/api/oauth/wizard/client-metadata

With CIMD, when the wizard hits EU for the first time, PostHog EU will fetch the metadata from the URL, auto-create the OAuthApplication, and the wizard works - no manual admin setup per region.

What needs to happen

1. Update WIZARD_CLIENT_ID in src/utils/provisioning.ts to use the CIMD metadata URL
2. Ensure the provisioning API on posthog side supports CIMD URLs (depends on PostHog/posthog issue for CIMD provisioning auth)
3. Test on both US and EU

Depends on

• CIMD provisioning auth support on posthog side
• CIMD metadata endpoint (#54975) ✅